This invention relates to a sodium ionization detector for detecting leaks of liquid sodium as a coolant for fast breeder nuclear reactors.
The conventional sodium ionization detector (hereinafter abbreviated to SID) comprises a transformer, an A.C. high power supply connected to the primary winding of the transformer, a filament connected to the secondary winding of the transformer, a collector located in facing relation with the filament so as to collect ionized sodium, a D.C. power supply for applying D.C. voltage between the collector and the filament, and an ammeter connected to the collector so as to measure ion current.
The sodium leaking from a sodium carrying pipe and the equipment associated therewith is ionized through a physical process called "surface ionization" at the filament surface. These sodium ions are collected at the collector by the potential difference provided between the collector and the filament. The ion current flowing through the collector is measured by the ammeter. Thus, the sodium leak from the sodium carrying pipe is detected. It has been known that the ion current is given by the following equation: ##EQU1## where e: a single electric charge (Coulomb)
I: ionized energy of sodium (eV) PA1 .psi.: work function of the filament metal (eV) PA1 k: Boltzmann's constant PA1 T: filament temperature (.degree.K) PA1 n: the number of sodium atoms impinging on a unit area of the filament per unit time (pieces cm.sup.-2 .multidot.S.sup.-1) PA1 S: filament surface area (cm.sup.2)
The above-mentioned SID involves a limitation of detection due to the effects of dark currents. Dark currents may be classified into three types; the first type of dark current is a leakage current flowing through insulators between electrodes, the second type is an ion current attributed to contamination on the filament surface and to the alkali metals which have been undesirably mixed in the filament material during the manufacturing process, and the third type is a dark current due to radiation.
The first type of dark current or leakage current can be reduced to less than 10.sup.-12 A by increasing the insulation resistance. However, the second type of dark current or ion current cannot substantially be reduced when a metal having a low melting point (e.g. below 2500.degree. C.) is used. In particular, this type of dark current tends to increase with time, and in the case of platinum, a dark current of 1 to 3 nA will increase up to 5 to 20 nA after the filament is exposed to a sodium containing gas for several hours. Thus, the capability of detection is degraded with time. Finally, an increase in the third dark current is also unavoidable unless radioactivity is decreased.
A sensor for detecting sodium leaks by measuring the sodium ion current by means of an ammeter is disclosed, for example, in U.S. Pat. No. 4,117,396.